General
The phenomenon of superconductivity was discovered in 1908 by Dutch Physicist Kamberlign Onnes, while studying the electrical resistance properties of pure mercury at very low temperatures. A superconducting material is one that when cooled below its critical transition temperature (Tc) will lose all it measurable electrical resistance. In 1933, Meissner and Oschenfield discovered that superconductors not only have zero electrical resistance, but also behave like perfect diamagnets. Superconductors are classified into two categories depending upon their magnetization properties. In an applied magnetic field, Type-I superconductors undergo a reversible thermodynamic transition from the perfectly diamagnetic superconducting state to the normal resistive state. Type II superconductors undergo two irreversible thermodynamic transitions. The first occurs at a lower critical field Hc1, and is a transition from a perfectly diamagnetic superconducting state to a “mixed” or vortex state. The second occurs at an upper critical field Hc2, and is a transition from the mixed state to the resistive normal state. In the mixed state, quantized units of magnetic field known as fluxoids are allowed to penetrate the superconducting material, while the bulk material maintains its diamagnetism. When a superconducting material is in its mixed state with fluxoids penetrating the material and a transport current is passed through the material, a Lorentz force is developed between the fluxoid and the transport current. If the fluxoid in not “pinned” to the superconducting material then it will move under this Lorentz force causing unwanted dissipation. A key to fabricating a practical superconducting is to have the “pinning” force large enough to withstand the Lorentz force from significant current flow. There are several known methods to increase pinning forces in superconductors each pertaining to the introduction of defects into the materials. Some known methods include physical defects, chemical defects, irradiation, etc., and can be found in prior artwork: U.S. Pat. No. 4,996,192 by Fleisher et al., 2) U.S. Pat. No. 5,034,373 by Smith et al., and U.S. Pat. No. 5,292,716 by Saki et al.
For any superconducting material there is a maximum or critical current density (Jc) that the material is able to conduct, a maximum or critical magnetic field (Bc) that can be applied, and a maximum or critical temperature (Tc) that the material can experience, without developing resistance. These three critical parameters of a superconductor are all interrelated and each play a crucial role in developing a practical material that can be used in real world applications. For example, in an externally applied magnetic field (H), the critical current density Jc (T, H) of a superconductor will decrease with increasing applied field. Similarly, the critical current density Jc (T, H) will decrease with increasing temperature up to the transition temperature Tc, where the material will revert back to its normal state. Once again, for practical applications where high critical current density is required, it is important to increase the pinning forces through the introduction of defects such as chemical doping, irradiation, or other physical deformation. For superconducting materials that possess anisotropic superconducting properties, it is additionally important to have a high degree of crystal texture to minimize “weak links” which can develop between the grain boundaries (see section below and claim 1).
High Temperature Superconductors and Low Temperature Superconductors
Until the 1986, all known superconducting materials had critical transition temperatures below˜23 K. This class of superconductors is commonly referred to as Low Temperature Superconductors (LTS) and typically consist of many metallic and inter-metallic compounds (e.g. Nb, Va, Hg, Pb, NbTi, Nb3Sn, Nb3Al, Nb3Ge, etc.). The fundamental quantum physics that governs all LTS materials is based on phonon mediated superconductivity.
In 1986, a new class of materials based upon oxide superconductors was discovered. This class of materials had significantly higher transition temperatures. They are commonly referred to as High Temperatures Superconductor (HTS) with some examples including (Re)—Ba—Cu—O, Bi—Sr—Ca—Cu—O, (Bi, Pb)—Sr—Ca—Cu—O, Tl—Ba—Ca—Cu—O, and Hg—Sr—Ca—Cu—O. The fundamental quantum physics that governs HTS materials is still not yet known.
Magnesium Di-boride
Superconductivity in the compound magnesium di-boride (MgB2) was recently discovered in February 2001. MgB2 has a superconducting transition temperature (Tc) of ˜39 K in zero applied field. MgB2 is difficult to classify as an LTS or HTS material based upon its transition temperature alone. From the technical literature, MgB2 appears to have a significant isotope effect, indicating a phonon mediated superconducting mechanism. Thus, MgB2 appears to be the ultimate strong coupling LTS material. The MgB2 material is crystalline/polycrystalline in nature and requires very high reaction temperatures, typically >600° C., to form the superconducting phase. The superconducting phase of this material has a hexagonal crystal structure. Unlike many of the metallic and inter-metallic LTS superconductors, which have isotropic superconducting properties, MgB2 has anisotropic superconducting properties. In this sense, MgB2 is similar to HTS materials, which possess highly anisotropic superconducting properties. Although it is still quite early in the development of practical MgB2 wire or cable, it appears that the highest quality, highest critical current material is obtained when MgB2 has some reasonable crystallographic alignment. Unlike its HTS counter-part which needs nearly perfect epitaxy to carry significant amounts of current, MgB2 requires some degree of texture of the crystal axis. This invention exploits this with the use of appropriate high temperature fiber substrates and (optional) buffer layer materials to obtain crystallographic alignment. One of the most critical factors in producing high quality, high Jc material is having good c-axis alignment of the MgB2 crystal.
Most of the early research on the MgB2 compound has been in the form of chemical doping to alter the superconducting properties (i.e. Tc, Jc, and Bc). Fortunately, the invention by Rey can is quite versatile and can implement the highest quality magnesium di-boride compound and any potential future chemically doped variants.
First Generation Bi-Oxide Conductors
First generation HTS wire and tape has been primarily limited to the Bi-oxide family because of its superior texturing properties. First generation, Bi-oxide based HTS wire and tape is almost exclusively fabricated with traditional “metallurgical” processes. The most common metallurgical process used to fabricate Bi-oxide wire and tape is the power-in-tube (PIT) method (see for example U.S. Pat. No. 5,106,825 by Mandigo et al.). However, there are several disadvantages to the PIT approach. The PIT method is expensive to fabricate and difficult manufacture. A typical figure of merit for a first generation Bi-oxide PIT wire or tape ranges from $50 to $300 per kA-m. The desired figure of merit is for any HTS wire is <$10 per kA-m. A fundamental limitation of the PIT approach is the use of silver or silver alloys as the containment medium. These materials, while chemically compatible, are expensive (˜$3-5 per kA-m) relative to the ultimate desired cost of the superconducting wire. The primary technical obstacle to practical implementation of the first generation Bi-oxide based material is its relatively moderate current carrying capacity at elevated temperatures (>60 K) and high magnetic fields (>1 T). The practical use of the Bi-oxide material appears to be intrinsically limited to lower temperatures (<40 K), low bending strains (<0.2%) and low magnetic fields (<2-3 T).
Another subtler disadvantage of this approach is the use of a substrate with planer (i.e. flat) geometry. Substrates with planer (flat) geometry suffer from two inherent disadvantages. First, they have higher eddy current loss when a magnetic field is applied perpendicular the face of the tape. This situation is unavoidable in many applications. Second, they generate a non-uniform self magnetic field. This will result in non-uniform current distribution in the superconducting material. Non-uniform current distributions result in an inefficient current flow, and thus, an uneconomical use of the superconducting material.
HTS Thin Films on Rigid Crystal Wafers
Until 1996, most HTS films were fabricated using traditional thick and thin film techniques for use in high frequency electronic device applications. Typical thick film techniques include sol-gel, dip coating, spin coating, electroplating, etc. Typical thin film techniques include rf/dc sputtering, co-evaporation, CVD, PVD, laser ablation, etc. Using these known film deposition techniques, very high quality HTS films with Jc>106 A/cm2 (77 K, self-field) were fabricated (see for example U.S. Pat. No. 5,231,074 by Cima et al). The primary reason for this success was that the HTS films were deposited on single crystal substrates that possessed a “natural” textured crystal structure orientation. Some typical single crystal substrates that have been used successfully to deposit texture HTS films are: sapphire (Al2O3), magnesium oxide (MgO), lanthanum aluminate (LaAlO3), strontium titinate (SrTiO3), as well as several others. The key to high quality HTS films once again being this natural highly oriented crystal structure template. By depositing the HTS films on highly oriented crystalline substrate templates, the HTS crystals themselves could grow in a highly textured format. With this high degree of crystal texture, HTS films will carry in excess of >106 A/cm2 at 77K, self-field. When HTS crystals are randomly aligned i.e. polycrystalline, they will have extremely low critical current densities. Low critical current densities are not useful in most real world device applications. For example, when HTS material is deposited on polycrystalline (i.e. no texture) metallic substrates (e.g. Ni, or Ni alloy), the result is a very poor quality HTS film with very low Jc's. Although high quality, high Jc HTS films could be grown quite readily on rigid crystalline substrates for use in electronic device applications (e.g. cavities, high frequency filters, mixers, etc.), they could not be fabricated into long lengths, which are necessary for most magnet applications (e.g. motors, generators, magnets, transformers, cables, etc.).
The goal for HTS conductors has been to reproduce the excellent superconducting properties obtained on the rigid crystalline wafers on a flexible substrates. U.S. Pat. No. 5,814,262 by Ketcham et al. teaches the process of fabricating thin inorganic sintered structures having strength and flexibility sufficient to permit bending without breakage in at least one direction to a radius of curvature of less than 20 centimeters.
Second Generation Coated Conductors
Oxide based HTS materials tend to have strong spatial anisotropic critical current and critical magnetic fields, while most of the metallic/inter-metallic LTS materials tend to have isotropic critical current and critical magnetic field properties. The existence of this strong anisotropy in HTS materials has led the development of very specific fabrication methods, including the second generation coated conductors. Second generation coated conductors use external means (i.e. not natural crystal structure) to introduce texturing to a substrate template. Films of non-superconducting buffer layers and superconducting layers are deposited in a highly controlled environment onto this textured substrate template for the specific purpose of subsequently growing HTS films with a high degree of in-plane crystal orientation. There are several known methods used to fabricate second generation HTS coated conductor including: rolling assisted bi-axial textures substrates (RABiTS), ion assisted beam deposition (IBAD), inclined substrate deposition (ISD), etc.
In 1996, researchers began to introduce thick/thin film deposition methods for fabricating long length coated conductors on flat (polycrystalline) metallic substrates. The metal of choice was typically Ni or one of its alloys, because of its ability to tolerate the high reaction temperature (>700° C.) necessary for HTS phase formation, yet remain chemically inert. Typically, metals have a polycrystalline order and directly depositing HTS materials on them would result in poor quality, low Jc films. The key to fabricating high quality, high Jc material on metallic substrates was the imparting of an “external” texturing means to either the template itself (e.g. RABiTS) or imparting a texturing means by the deposition process itself (e.g. IBAD, PACVD, ISD, ITEX). Several of the known methods for imparting texture to the HTS materials (IBAD, RABiTS, PACVD, ISD, ITEX), are known to produce high quality, high Jc coated conductor.
Applications of Superconducting Wire
Copper, aluminum, and magnetic iron are the primary conventional materials of devices used in today's electrical power sector. A long time challenge in the electrical power industry has been to make practical, economic superconducting wire. There are several potential applications of superconducting wire in the electric power industry including: ac/dc transmission cables, ac/dc motors, magnets, transformers, generators, energy storage devices (SMES), fault current limiters (FCL's), etc. Superconducting wire also has applications is several other industrial applications as well including: MRI, NMR, magnetic separation, waste remediation, particle accelerators, fusion reactors, ship propulsion, etc.